An amine compound refers to a compound, such as an ammonia molecule, containing a nitrogen atom having an unshared electron pair and is typically basic. Such an amine compound has been found from both plants and animals. Amine compounds extracted from plants include nicotine contained in tobacco and cocaine which can cause hallucination. Many of daily used drugs contain an amine functional group. A typical drug is penicillin. Further, dopamine well known as a material that stimulates brain nerves and phenylethylamine as a main substance of chocolate are typical compounds each containing an amine functional group. In many cases, amine compounds each having a relatively small molecular weight exist in a liquid-phase form at room temperature under atmospheric pressure, give out smell of fish, and cause skin allergies. Further, such liquid amines easily react with oxygen or the like in the air so as to be changed, and, thus, if they are not completely sealed, it is difficult to maintain purity thereof. Therefore, liquid amines are difficult to handle and store and are limited in applications thereof.
As a method for solving the above problem, a solid amine salt prepared by reacting an amine with an acid has been used in substitution for amine. Such an amine salt is prepared as a solid salt by adding a strong acid such as sulfuric acid or hydrochloric acid to a liquid amine with a precipitation reaction. By way of example, when a hydrochloric acid solution is put into phenylethylamine, a precipitation reaction occurs in an instant with conversion into phenylethylamine hydrochloric acid which has been actually used in substitution for liquid amine. Such solid amine salts exist in a stable solid form at room temperature and have characteristics similar to those of liquid amines when they are dispersed in a solution and react with other compounds. Further, such salts have a very high solubility to water and are useful for utilizing amine. In particular, a drug containing an amine functional group which is not dissolved well in water due to its high molecular weight is used as being bonded to an acid such as hydrochloric acid. This is because if the drug is converted into a salt, solubility is sharply increased in an aqueous solution and thus the drug is suitable for use as a drug.
However, such amine salts need a solvent when being generated, the solvent needs to be removed after a reaction due to unnecessary materials added to generate the salts, and an additional process for removing a remaining material is needed. Further, a neutralization process for removing a strong acid such as hydrochloric acid or sulfuric acid used in a process for preparing an amine salt is essential. Such a multistep process causes environmental pollution.
As described above, as one of alternatives to solve such problems of the liquid amine and the solid amine salt, there is a method for preparing solid amine derivatives from a liquid amine by using eco-friendly carbon dioxide as a reactant. If a reaction is made between a liquid amine and carbon dioxide, the carbon dioxide is generally inserted into N—H bonds of amine molecules and converted into carbamic acid derivatives. Most of these carbamic acid derivatives exist in a stable solid form in the air. Further, they can be easily separated into the amine and the carbon dioxide by a slight external change, and, thus, they can act as a good alternative to solve the problems of the liquid amine and the solid amine salt.
Many methods for preparing new compounds using carbon dioxide as a reactant have been known so far, and various methods for preparing new compounds by the methods have been reported [Sakakura, T.; Choi, J.-C.; Yasyda, H., Chem. Rev., 2007, 107, 2365]. Further, it is known that carbamic acid derivatives can be formed by a reaction between an amine and carbon dioxide. However, in most of the conventionally known reactions between an amine and carbon dioxide, while carbon dioxide gas is discharged under atmospheric pressure for a long time by using a solvent, a compound is formed in a reactor and used, without an additional separation process, for an intermediate step in a process for forming another compound, or a compound is used for synthesizing a compound in a gel form (U.S. Pat. No. 3,551,226), or there are known researches in which a compound is formed by using a polar solvent as an ionic liquid [1) Jessop, P. G.; Heldebrant, D. J.; Li, X.; Eckert, C. A.; Liotta, C. L., Nature, 2005, 436, 1102, 2) Lam Phan; Andreatta J. R.; Horvey, L. K.; Edie, C. F.; Luco. Aimé e-L.; Mirchandani, A.; Darensbourg, D. J.; Jessop, P. G., J. Org. Chem., 2008, 73, 127-132, 3) Liu, Y.; Tang, Y.; Barashkov, N. N.; Irgibaeva, I. S.; Y. Lam, J.; Hu, W. R.; Birimzhanova, D.; Yu, Y.; Tang, B. Z., J. Am. Chem. Soc., 2010, 132, 13951].
In U.S. Pat. No. 3,551,226, it is disclosed that liquid amines react with carbon dioxide under atmospheric pressure at a low temperature for a long time and are converted into and used as a gel type compound. Such a gel type compound contains amine and carbon dioxide at an irregular ratio and is a very sticky liquid, and, thus, it is difficult to separate the gel type compound into a pure solid amine in powder form by a typical drying method. Further, if an amine is solidified by using carbon dioxide under atmospheric pressure, a sticky gel type compound is obtained rather than a solid compound powder. As described above, it is known that carbamic acid derivatives can be formed by a reaction between an amine and carbon dioxide. However, there is no research result regarding a separation or preparation method for a solid carbamic acid derivative in powder form.